On the composition of ammonia–sulfuric-acid ion clusters during aerosol particle formation

The formation of particles from precursor vapors is an important source of atmospheric aerosol. Research at the Cosmics Leaving OUtdoor Droplets (CLOUD) facility at CERN tries to elucidate which vapors are responsible for this new-particle formation, and how in detail it proceeds. Initial measurement campaigns at the CLOUD stainless-steel aerosol chamber focused on investigating particle formation from ammonia (NH sub(3)) and sulfuric acid (H sub(2)SO sub(4)). Experiments were conducted in the presence of water, ozone and sulfur dioxide. Contaminant trace gases were suppressed at the technological limit. For this study, we mapped out the compositions of small NH sub(3)-H sub(2)SO sub(4) clusters over a wide range of atmospherically relevant environmental conditions. We covered [NH sub(3)] in the range from < 2 to 1400 pptv, [H sub(2)SO sub(4)] from 3.3 x 10 super(6) to 1.4 x 10 super(9) cm super(-3) (0.1 to 56 pptv), and a temperature range from -25 to +20 degree C. Negatively and positively charged clusters were directly measured by an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer, as they initially formed from gas-phase NH sub(3) and H sub(2)SO sub(4), and then grew to larger clusters containing more than 50 molecules of NH sub(3) and H sub(2)SO sub(4), corresponding to mobility-equivalent diameters greater than 2 nm. Water molecules evaporate from these clusters during sampling and are not observed. We found that the composition of the NH sub(3)-H sub(2)SO sub(4) clusters is primarily determined by the ratio of gas-phase concentrations [NH sub(3)] / [H sub(2)SO sub(4)], as well as by temperature. Pure binary H sub(2)O-H sub(2)SO sub(4) clusters (observed as clusters of only H sub(2)SO sub(4)) only form at [NH sub(3)] / [H sub(2)SO sub(4)] < 0.1 to 1. For larger values of [NH sub(3)] / [H sub(2)SO sub(4)], the composition of NH sub(3)-H sub(2)SO sub(4) clusters was characterized by the number of NH sub(3) molecules m added for each added H sub(2)SO sub(4) molecule n ( Delta m/ Delta n), where n is in the range 4-18 (negatively charged clusters) or 1-17 (positively charged clusters). For negatively charged clusters, Delta m/ Delta n saturated between 1 and 1.4 for [NH sub(3)] / [H sub(2)SO sub(4)] > 10. Positively charged clusters grew on average by Delta m/ Delta n = 1.05 and were only observed at sufficiently high [NH sub(3)] / [H sub(2)SO sub(4)]. The H sub(2)SO sub(4) molecules of these clusters are partially neutralized by